Candidate. Dr. Sabnis is a pediatric oncologist with training in genetics and molecular biology whose long-term goal is to lead an independent laboratory defining and targeting vulnerabilities in the protein homeostasis networks of pediatric sarcomas. He has begun to establish himself as a leader in this field through a first author publication in PNAS detailing HSP70 dependence in rhabdomyosarcoma (RMS) and contributing authorship on publications in Nature Medicine and Nature Genetics. This K08 award will be a critical vehicle for his ongoing career development, providing key mentorship and instruction in 1) designing preclinical trials to enable clinical translation of bench research, 2) probing the ER quality control mechanisms cancer cells subvert to support their survival, and 3) using genetic manipulation and cell biologic readouts to interrogate proteostasis in sarcoma biology. Environment. UCSF is an outstanding research environment with 1300 principal investigators and over $500 million in support from the NIH (ranking 2nd among all institutions). Two co-mentors will help Dr. Sabnis achieve his aims. Dr. Trever Bivona, MD PhD, is a medical oncologist with expertise in biologically defining rational polytherapy for oncogene-driven solid tumors. Dr. Bivona has extensive research support including an NIH Innovator?s Award and several R01s, and has mentored five post-doctoral fellows into independent positions in the last five years. Dr. Sabnis will be co-mentored by Dr. Kevin Shannon, MD, a pediatric oncologist who has been the primary mentor for multiple K-series award recipients from the NCI. Dr. Sabnis will also meet semi- annually with a mentoring committee, comprised of Dr. Bivona; Dr. Shannon; Dr. Jonathan Weissman, an expert in ER quality control and mentor to many K-supported trainees; and Dr. Kate Matthay, a pre-eminent pediatric oncology clinical researcher who will support the clinical translation of his discoveries. Research. High-risk RMS patients have dismal outcomes despite maximally intensified chemotherapy, highlighting a need for new, biology-driven treatments. We found that inhibiting the cytosolic protein chaperone HSP70 lethally activates the unfolded protein response (UPR) in RMS, but not in other cancers. I hypothesize that RMS cells rely on HSP70, acting together with its co-chaperone DNAJC17 and the ATPase p97, to lower ER protein load through ER-associated degradation (ERAD). ERAD inhibition in RMS thus defines a novel therapeutic strategy. In aim 1, we will test the pharmacologic parameters and efficacy of two drugs that disrupt ER quality control in murine RMS models. In aim 2, we will identify the structural domains of DNAJC17 that are necessary to maintain ER homeostasis, and test the hypothesis that this HSP70-DNAJC17-p97 axis enables ERAD and thereby ensures RMS cell survival. These aims will provide crucial molecular detail into the basis of the RMS-specific lethality of HSP70 inhibition we discovered. Overall, this work will catalyze a broader effort to discover therapeutic targets in sarcoma proteostasis network that will be the basis for future R01 proposals. !